Fritz Lang's The Testament of Dr. Mabuse (Das Testament des Dr. Mabuse) is the story of a police inspector's attempt to stop some criminals who are counterfeiting money and murdering people. The inspection gets kind of weird as the criminals seem to be ordered around by a guy named Dr. Mabuse who is in an insane asylum and will not speak or do much of anything. This film is actually the second film in a trilogy of films pertaining to Dr. Mabuse, but the other two films aren't particularly famous. I thought this was pretty interesting although the ending seemed kind of underwhelming.
For an early sound film, this film is particularly noteworthy for its rather sophisticated use of sound. Like Lang's previous film, M the sound techniques used in this film are rather advanced. Both films make use of leitmotif, a recurring musical theme pertaining to a particular place, person, or idea. The film also uses sound that is misidentified by the audience (a pocketwatch spring unwinding, intended to simulate a telephone's ring; a gunshot masked by the sound of car horns; the ticking of a bomb changing to a spoon tapping on an eggshell). The film also had some pretty advanced special effects considering the time it was made.
While this film is certainly pretty good, it does not live up to Lang's M which is a vastly superior film. This may be more enjoyable than Lang's Metropolis, but realistically it's not as good as that film either. This is definitely worth viewing, but this won't go down as one of cinema's masterpieces like the aforementioned films.